This invention relates to the manufacture of semiconductor devices with a depletable multiple-region semiconductor material that provides a voltage-sustaining space-charge zone when depleted, and to a method of fabricating such a material. The invention also relates to other semiconductor material and semiconductor devices produced by such methods.
The voltage-sustaining space-charge zone results from charge-carrier depletion of interposed p-type and n-type regions that form multiple p-n junctions in the material. The intermediate dimensions (width or thickness) of the interposed p-type and n-type regions need to be small enough (in relation to their dopant concentrations) to allow depletion of the region across its intermediate dimension without the resulting electric field reaching the critical field strength at which avalanche breakdown would occur in that semiconductor. This is an extension of the famous RESURF principle. Thus, the depletable multiple-region material may be termed “multiple p-n RESURF” material. In the voltage-sustaining zone formed of first regions of one conductivity type interposed with second regions of the opposite conductivity type, the dopant concentration and dimensions of the first and second regions are such that (when depleted in a high voltage mode of operation) the space charge per unit area in the first and second regions balances at least to the extent that the electric field resulting from the space charge is less than the critical field strength at which avalanche breakdown would occur in that zone.
The photovoltaic solar cell industry is extremely cost sensitive, and the cost of a starting silicon wafer is typically nearly half of the value of the finished photovoltaic module. Thus, in this industry it is extremely important that the silicon wafers are used as efficiently as possible. Most photovoltaic solar cells are manufactured by processes on the major surfaces of the silicon wafer, resulting in a depletion zone(s) parallel to the major surfaces. Efficiency of the conversion of light to electron-hole pairs is maximized at the depth of the depletion zone, but is considerably less at other depths. High-purity silicon crystal offsets some of these losses by providing an extended diffusion length, but is more expensive to produce and still has deep regions that poorly contribute to electron-hole pair conversion. Depth of light conversion is also strongly related to photon energy (wavelength), resulting in losses of otherwise useful parts of the solar spectrum.
U.S. Pat. No. 6,703,292 (Grover) discloses a method for producing semiconductor devices with depletable multiple-region (multiple p-n junction RESURF) semiconductor material comprising alternating p-type and n-type regions which utilizes patterned Neutron Transmutation Doping (NTD). Grover's method is an improvement over previous procedures, and uses a collimated beam of thermal neutrons and a neutron-absorbing mask in close proximity to the semiconductor material. Two problems exist for Grover's method; one, thermal neutrons are not easily collimated, and two, masks have large feature sizes as a result of their manufacturing methods. Such large feature sizes are carried into the semiconductor material and exacerbated by diffraction effects at the edges of features in the proximity mask. Similar limitations occur in the exposure step in x-ray proximity lithography, which has most of the same elements as described in Grover's method.
The standard approach for collimating x-rays used at synchrotron facilities, which is to provide great distance from a small origin to the work necessary for proximity lithography, is not effective for thermal neutrons. Thermal neutrons are “produced” in moderating material, such that fast neutrons are slowed to thermal energies via scattering reactions. Consequently, the neutron flux is distributed throughout the moderator and any neutrons leaving the moderator do so with random angular direction. Collimating the exiting thermal neutrons via transmission grids and/or a reduction apertures results in a significant flux reduction. Additionally, thermal neutrons are relatively slow moving, heavy particles with flight paths that are affected by gravity. Grover does not state any method for producing a collimated beam of thermal neutrons sufficient to transmute silicon atoms to phosphorus in a pattern, and it is unclear if the invention has ever been practiced.
In contrast to the exposure step in proximity lithography, which is similar to Grover's method for patterned neutron transmutation doping, exposures in projection lithography are controlled by focusing optics. In this more common process, ultraviolet radiation exiting absorbing masks with large features sizes are projected and demagnified by optical components. In addition to the obvious benefit of imprinting small features, the projection method also reduces the requirements for collimated illumination and controls for mask-edge diffraction effects. It is standard practice in optical projection lithography to have multiple exposures to produce a precise pattern. Prior to this disclosure, no neutron optical projection lithography methods have been proposed for Neutron Transmutation Doping (NTD), a common bulk material doping process.
Photovoltaic (PV) cells are semiconductor devices that convert light to electrical voltage, and are generally made of doped silicon material. Typically, p-type doped silicon is produced as a wafer or substrate onto which n-type doped silicon is deposited. A depletion zone forms in the region of the p-n junction, as discussed above. Photons of light that are absorbed in the depletion zone contribute to the cell's electrical current at nearly 100% probability, as the electron-hole pair are quickly swept apart by the electric field and are collected. Away from the junction, the collection probability drops off. If the carrier is generated more than a diffusion length away from the junction, then the collection probability of this carrier is quite low. Unfortunately, most photons are absorbed in regions that are shallower or deeper than the depletion zone and overall light-to-electricity efficiency is reduced.
The nature and magnitude of a material's band structure are parameters which influence the electronic and optoelectronic devices fabricated therefrom. For example, diodes made from semiconductors with wide bandgaps will tend to have higher breakdown voltages because these materials will have fewer thermally-generated charge carriers at any given temperature and therefore will be less susceptible to avalanche effects. Gallium arsenide will be a material of choice for radiation-generating devices because it has a direct bandgap. Silicon, on the other hand, has been considered fundamentally unsuitable for use as an emitter of radiation. This is because silicon is an indirect bandgap material in which fast, non-radiative recombination processes completely dominate the much slower radiative recombination processes. Indeed in bulk silicon, at room temperature, radiation emission is almost entirely absent.
With the continuing and rapid development of computer processors, the constant demand for increased processing power and speed and reduced size necessitates an ever increasing complexity of the interconnecting metallisations. It is anticipated that this complexity will eventually present an insurmountable obstacle to further development (the breakdown of Moore's Law) because electrons will spend a disproportionate amount of time in the metallisations instead of in the components they interconnect, thereby curtailing processing power and speed.
Optoelectronic circuits based on silicon technology offer a way forward because optical coupling is many orders of magnitude faster than connections based on the diffusion of charge carriers. However, this approach requires development of an efficient room temperature radiation-emissive device based on silicon. Clearly, such a device could be used to enhance the functionality of other silicon devices and could lead to implementation of all-silicon integrated optoelectronic systems.
Prior techniques for making a silicon-based optoelectronic device include porous or nano-particle silicon, or multilayer compound semiconductors, either of which is not bulk silicon. Some attempts of ion implantation to form strain fields within silicon have had limited success, with poor focusing at higher energy/penetration depths being the main drawback (Homewood et al, U.S. Pat. No. 7,274,041).
It is obvious that a method is needed for producing closely matched and closely spaced p and n type doping for vertical devices. It is also obvious that more economical vertical devices manufactured from less pure silicon crystal, polycrystalline silicon, or hydrogenated amorphous silicon is needed. It is also obvious that a more efficient photovoltaic cell is needed, particularly a photovoltaic cell that has uniformly high contribution to current for carriers created at all vertical depths within the semiconductor material. It is also obvious that optoelectronic devices manufactured from silicon crystal would be advantageous.